1. Field of the Invention
The present invention relates to a fixing device for an image forming apparatus such as a copier, printer, facsimile machine, or similar electrophotographic image forming apparatus. More particularly, the invention is concerned with setting the proper temperature of a fixing roller and pressure roller to produce high quality images.
2. Discussion of the Background
A background image forming apparatus includes a heat roller and an additional roller between which a sheet is fed in order to fix an image thereupon. The heating roller generally contains a heating mechanism which is used to heat the heating roller. In addition, heat sensors or the like are used to determine the temperature of the heating roller and this information is used to control a temperature of the heating roller. For example, in a full color copy mode, the temperature of the heater roller is set to a higher temperature than for a monochromatic copy mode.
FIG. 1 is a graph illustrating a reference environment temperature (ambient temperature) versus a set temperature of a fixing roller used by a background printer. The reference environment temperature is the temperature of the environment the image forming apparatus is operating in and is detected by a sensor or the like. For example, as shown in FIG. 1, when the reference temperature is 20.degree. C. (ambient temperature), the set temperature of the fixing roller in a full color mode (line Ta) is 160.degree. C. Similarly, for a reference temperature of 20.degree. C. the set temperature of the fixing roller in a monochromatic mode or a thick paper mode (line Text) is 140.degree. C. If the reference temperature was to increase, it can be seen in FIG. 1 that the slopes of lines Ta and Text decrease, and therefore the set temperature of the fixing roller is decreased. The line Ta shows a relationship between the reference environment temperature and a set temperature of the fixing roller for a full color mode and the line Text shows the same relationship for a monochromatic or thick paper mode. The line Twait is the temperature of the fixing roller set for a standby state, that is, when the apparatus is not in a copy mode. The set temperatures of the lines Ta and Twait are predetermined by a mathematical procedure based on the reference environment temperature.
In order to prevent improper fixing from occurring in the full color mode, a difference of temperature between a highest and lowest temperature .DELTA.T equals 16.degree.. In this case, the highest temperature for the full color mode would be 168.degree. C. and the lowest temperature would be 152.degree. C. However, as is shown in FIG. 1, the background device also sets the monochromatic or thick paper mode temperature gradient to 16.degree.. Consequently, when the reference environment temperature is relatively low, that is below the ambient temperature of 20.degree.0 C., the set temperature of the fixing roller is increased. This creates a problem in that the temperature is increased too much which results in an offset or luster appearance on the final image. On the other hand, if the reference temperature increases (e.g., above 20.degree. C.), the set temperature is decreased too much resulting in an improper fixing of the image for the monochromatic or thick paper copying mode.
In addition, in the case of the background copier apparatus, when the temperature sensor is out of order or not functioning properly, the copying process must be halted and the temperature sensor replaced.